Acoustic resonators can be used to implement signal processing functions in various electronic applications. For example, some cellular phones and other communication devices use acoustic resonators to implement frequency filters for transmitted and/or received signals. Several different types of acoustic resonators can be used according to different applications, with examples including bulk acoustic wave (BAW) resonators such as thin film bulk acoustic resonators (FBARs), coupled resonator filters (CRFs), stacked bulk acoustic resonators (SBARs), double bulk acoustic resonators (DBARs), and solidly mounted resonators (SMRs). An FBAR, for example, includes a piezoelectric layer between a bottom (first) electrode and a top (second) electrode over a cavity. BAW resonators may be used in a wide variety of electronic applications and devices, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs operating at frequencies close to their fundamental resonance frequencies may be used as a key component of radio frequency (RF) filters and duplexers in mobile devices.
FIG. 1 is a block diagram depicting a conventional acoustic resonator. Referring to FIG. 1, acoustic resonator 100 includes a piezoelectric layer 130 of piezoelectric material applied to a top surface of a bottom electrode 110, and a top electrode 140 applied to a top surface of the piezoelectric layer 130, resulting in a structure referred to as an acoustic stack. The acoustic stack is formed on a substrate 105 over a cavity 110 in the substrate. A planarization 120 layer may be included to provide a planarized surface on which to apply the piezoelectric layer. Where an input electrical signal is applied between the electrodes, reciprocal or inverse piezoelectric effect causes the acoustic stack to mechanically expand or contract depending on the polarization of the piezoelectric material. As the input electrical signal varies over time, expansion and contraction of the acoustic stack produces acoustic waves that propagate through the acoustic resonator in various directions and are converted into an output electrical signal by the piezoelectric effect. Some of the acoustic waves achieve resonance across the acoustic stack, with the resonant frequency being determined by factors such as the materials, dimensions, and operating conditions of the acoustic stack. These and other mechanical characteristics of the acoustic resonator determine its frequency response.
Generally, a conventional FBAR, such as acoustic resonator 100, may be designed to operate at high frequencies, such as approximately 3.6 GHz, for example. In this case, each of the bottom resonator 110 and the top resonator 140 would be formed of tungsten (W) approximately 2700 Å thick top, and the piezoelectric layer 130 would be formed of aluminum nitride (AlN) approximately 1600 Å thick. Conventionally, aggregate thickness of the acoustic stack is one half the wavelength λ (or λ/2) corresponding to the thickness extensional resonance frequency of the acoustic resonator 100.
FIG. 2 is a graph showing Normalized Peak Strain Energy (NPSE) distribution for Mason pseudo-mode across the acoustic resonator 100 in the vertical direction. The Mason pseudo-mode is motion excited by the vertical electric field in the active region of the acoustic resonator 100. Referring to FIG. 2, plot 210 shows normalized acoustic impedance to provide a position marker within the acoustic stack, and plot 320 shows the NPSE distribution. Plot 210 corresponds to the various elements of the acoustic stack, as indicated by the reference numbers between the vertical dashed lines. That is, from left to right, distinct sections of the normalized acoustic impedance correspond to the bottom electrode 110, the piezoelectric layer 130, and the top electrode 140, respectively. As shown by plot 220, the typical energy distribution for the λ/2 mode has one peak in the center of the piezoelectric layer 130 and two nulls at the metal/air surfaces (i.e., at the bottom edge (cavity 108) and the top edge of the acoustic resonator 100).
While the thickness of the bottom and top electrodes 110 and 140 may be sufficient for low series resistance, the very thin piezoelectric layer 130 (typical resonators for RF duplexers operating in 0.7 GH-2.5 GHz range would have piezoelectric layer thickness of about 5000 Å-20000 Å) poses a number of problems as outlined below.
Generally, a conventional acoustic resonator, such as acoustic resonator 100, suffers from several issues when designed for operation at high frequencies. For example, the acoustic resonator 100 would tend of have a low quality (Q) factor due to high series resistance Rs resulting from the relatively thin bottom and top electrodes 110 and 140. The acoustic resonator 100 would also tend to have low parallel resistance Rp due to the relatively thin piezoelectric layer 130, resulting in small area. Furthermore, the piezoelectric layer 100 would be susceptible to electro-static discharge (ESD) failures due to large electric fields, low RF power level failures due to the small area and resulting high RF-power density, and large perimeter-to-area loss due to small overall device area.
For example, acoustic resonators are generally designed to meet a specific characteristic electrical impedance Z0 requirement. The characteristic electrical impedance Z0 is proportional to the resonator area and inversely proportional to the desired frequency of operation and thickness of the piezoelectric layer. The thickness of the piezoelectric layer is predominantly determined by the desired frequency of operation, but also by the desired electromechanical coupling coefficient Kt2. Within applicable limits, the electromechanical coupling coefficient Kt2 is proportional to thickness of the piezoelectric layer and inversely proportional to thicknesses of the bottom and top electrodes. More specifically, the electromechanical coupling coefficient Kt2 is proportional to the fraction of acoustic energy stored in the piezoelectric layer and inversely proportional to the fraction of acoustic energy stored in the electrodes. Thus, for a predetermined impedance Z0, the resonator size, and therefore its cost, may be reduced by using piezoelectric material with higher intrinsic electromechanical coupling coefficient kt2 (for instance, aluminum nitride doped with scandium), as it allows use of a thinner piezoelectric layer (and therefore reduction of the resonator area) at the expense of increasing thicknesses of the bottom and top electrodes in order to maintain the desired resonance frequency. Therefore, as mentioned above, for high-frequency applications, specific electromechanical coupling coefficient Kt2, impedance Z0 and operating frequency requirements will enforce reduction of the active area and piezoelectric layer thickness, and the resulting reduction of the overall quality factor Q of the device and the robustness to ESD and high RF-power failures. Therefore approaches are needed to increase the device area and piezoelectric material thickness, while preserving electromechanical coupling coefficient Kt2, impedance Z0 and operating frequency as determined by a specific application.